(1) Field of the Invention
The present invention relates generally to rain gauges and more particularly to a method and apparatus for acoustically measuring rainfall.
(2) Description of the Prior Art
Prior art rain gauges measuring up to professional standards generally fall into four categories: (1) Tipping gauges, (2) Weighing gauges, (3) Optical gauges and (4) Capacitance gauges. The basic operating principles and inherent drawbacks of each of these gauges will be described briefly hereinbelow in order to provide a more clear understanding of the advantages of the present invention.
Tipping gauges are devices that collect precipitation by alternately filling two small buckets, each of which holds a precise amount of water. When one bucket fills, a mechanism allows the filled bucket to tip thereby emptying its contents. The number of bucket tips is counted with a mechanical or electrical counter. While this type of gauge is used extensively by the United States Weather Service, its disadvantages include: (1) high maintenance due to its mechanical nature, (2) its propensity to jam or malfunction during heavy downpours, and (3) the requirement that a firm and level mounting surface be used to maintain the system's stability and vertical position.
Weighing gauges are devices that simply collect precipitation in a bucket and convert the weight into curvillinear movement of a recording pen's inked trace on a moving paper chart. However, evaporation of the collected rainfall over time makes it difficult to interpret the chart recording. In addition, weighing gauges are relatively inaccurate in recording rainfall rates due to slow chart speeds. Weighing gauges are also mechanically complex and require periodic emptying of the bucket.
Optical gauges focus a horizontal light beam across a calibrated path length through which the precipitation falls. Typically, the light beam is generated by an infrared light emitting diode. Whenever the precipitation falls through this horizontal beam, a fluctuation of the beam results. The amount of fluctuation is proportional to the intensity of the precipitation rate, and is measured by analyzing the frequency of the fluctuation. The optical gauge does not accumulate the rainfall directly, but makes a measurement by electronically summing or integrating instantaneous rainfall rates. Accordingly, small errors in instantaneous rates are compounded into larger errors when measuring total rainfall amounts over an extended time. Additionally, its use is somewhat restricted due to its cost and complexity.
Capacitance gauges are devices that uses capacitance to measure the amount of rainfall collected in a water column. Specifically, the water column serves as the capacitor's dielectric material. As the water column rises, a DC voltage proportional to the amount of water in the column is indicative of the amount of precipitation. However, the opening of the water column that receives the precipitation is relatively small compared to other gauges. Accordingly, its accuracy is not well established. Furthermore, once the water column is filled, the entire contents must be siphoned off before it can again measure precipitation.
To summarize, the major disadvantages associated with the aforementioned rain gauges are cost, mechanical complexity and/or the need for frequent maintenance. However, the presence of any one of these disadvantages in a rain gauge makes it inappropriate for widespread use in one of the many thousands of remote locations around the world (on land or at sea). Unfortunately, none of the prior art methods or apparatus provides an inexpensive combination of steps or parts to achieve accurate, maintenance-free rainfall measurements.